French Patent Application FR-A-2,399,242 discloses a process for the preparation of liposomes in aqueous suspension, according to which a first aqueous phase, which may contain active substances, is dispersed in a solvent which is non-miscible or slightly miscible with water, in the presence of a lipid of the formula XY, where X is a hydrophilic group and Y is a lipophilic group, which makes it possible to obtain a continuous solvent phase of "liposome precursors" which consists of microscopic globules of aqueous phase whose envelope is a film of the substance XY; these "vesicle precursors" are emulsified in an aqueous medium in the presence of excess lipid XY or another lipid ZW, where Z is a hydrophilic group and W is a lipophilic group, the abovementioned solvent being removed before or after said emulsification. However, this process requires the use of sound or ultrasound waves for the dispersion of the first step, a technique which is poorly suited to productions on an industrial scale.
French Patent FR-B-2,418,023 discloses a process for the preparation of lipid, oligolamellar, synthetic capsules which are suitable to enclose a biologically active material (or materials), according to which a mixture of an organic phase which contains, in a solvent, a compound capable of forming capsule walls, with an aqueous phase which contains the biologically active material to be encapsulated is prepared, the ratio of the organic phase to the aqueous phase being such that it is suitable for providing an emulsion of the water-in-oil (W/O) type; a homogeneous emulsion of this type is formed; the organic solvent of the emulsion is evaporated until a mixture is obtained having a gel character; and this mixture is converted into a suspension of synthetic, oligolamellar capsules enclosing the biologically active material by dispersing it in an aqueous medium. However, in this process, the concentration of the lipids in the organic phase has to remain relatively low: the range indicated is 0.05 to 5% by weight, which does not make it possible to obtain in a simple manner compositions having a high concentration of vesicles. Furthermore, the emulsification is preferably achieved by ultrasound and thus hardly applicable on an industrial scale.
Japanese Patent Application 87/273,386 (published under the number 1,117,824) describes a process for the preparation of vesicles in which lipids are dissolved in an organic solvent, water is added, the solvent is removed to form a hydrated phase and this hydrated phase is dispersed in an aqueous phase to form vesicles. This process requires a treatment of the dispersion obtained in order to reduce the size of the vesicles and their size distribution, this treatment being carried out either by means of ultrasound or a homogenizer. The disadvantage of ultrasound has already been discussed; homogenization has the disadvantage, apart from the fact that it increases the temperature of the dispersion, which may be damaging to heat-sensitive encapsulated active substances, that it releases metallic particles into the medium, which may be damaging to encapsulated active substances which are sensitive to traces of metals.
French Patent Application FR-A-2,561,101 describes a process for the preparation of liposomes which consists in forming an emulsion of the W/O type by dispersing a first aqueous phase in an organic solvent which is immiscible with water in the presence of molecules comprising a lipophilic fraction and a hydrophilic fraction, then removing the major portion of the organic solvent from this W/O emulsion by a mild method, to give a concentrated W/O emulsion, and gradually adding to this emulsion with stirring a second aqueous phase which contains molecules comprising a lipophilic fraction and a hydrophilic fraction, the residual organic solvent being removed simultaneously by evaporation. In this case, the lipid concentration in the organic phase is also low: in this respect, the range given is 0.5 to 5% by weight. Moreover, the removal of the organic solvent in the second step is predominantly carried out by decanting, which requires a lot of time and is therefore detrimental to industrial application of this process.
Finally, European Patent Application EP-A-349,429 discloses a process for the preparation of dispersible colloidal systems of amphiphilic lipids in the form of sub-micron oligolamellar liposomes, whose wall is composed of these lipids and, optionally, a substance A, and whose core is composed of water or an aqueous solution optionally containing a substance B. According to this process, a liquid phase substantially composed of a solution of lipids (and, optionally, of the substance A) in a solvent which may contain the substance B in solution is prepared; a second liquid phase substantially composed of water or an aqueous solution of the substance B is prepared; the first phase is added to the second phase with moderate stirring such that virtually immediately a colloidal suspension of vesicles is obtained; if desired, all or a portion of the solvent and the water is removed so that a colloidal suspension of the required vesicle concentration is obtained. In practice, the solvents used are water-miscible solvents. The lipid concentration in the solvent given ranges from 0.1 to 10% by weight, which is very limited.